High pressure ion source for ion optical analytical equipment and for particle accelerators

ABSTRACT

A high pressure ion source having a metal gas inlet tube connected electrically at one end to the ion source and at the other end to a pressure reducing valve. The electron gun has its cathode separated from the ionization chamber by a beam transmitting system that includes a metallic screened hole near the cathode. The screened hole communicates with a vacuum exit tube. The ionization chamber includes a plurality of electrodes in the ion exit path. The space between the last electrode and the focusing electrodes is screened electrically from the chamber walls by a metallic grid. The gas inlet tube, pressure reducing valve, ionization chamber and the grid are maintained at the same potential.

United States Patent Knof et al.

[ Jan. 14, 1975 Wilhelm Hansen, both of Hamburg, all of Germany Dr. Hans Knot, Norderstedt, Germany Filed: Aug. 2, 1972 Appl. No.: 277,310

Assignee:

[30] Foreign Application Priority Data Aug. 31, 1971 Germany 2143460 US. Cl 313/231, 250/427, 313/63 Int. Cl. H0lj 39/34 Field of Search... 250/4l.9 G, 41.9 S, 41.9 SB;

lil l8 L ANODE TUBE Deren Negative lonen" by H. Knof et al. from Zeitschrift Fur Naturforschung, Vol. 25a, June, 1970, pages 849-852. Q3.Z4

Primary Examiner-William F. Lindquist Attorney, Agent, or FirmMorgan, Finnegan, Durham & Pine [57] ABSTRACT A high pressure ion source having a metal gas inlet tube connected electrically at one end to the ion source and at the other end to a pressure reducing valve. The electron gun has its cathode separated from the ionization chamber by a beam transmitting system that includes a metallic screened hole near the cathode. The screened hole communicates with a vacuum exit tube. The ionization chamber includes a plurality of electrodes in the ion exit path. The space between the last electrode and the focusing electrodes is screened electrically from the chamber walls by a metallic grid. The gas inlet tube, pressure reducing valve, ionization chamber and the grid are maintained at the same potential.

5 Claims, 2 Drawing Figures PRESSURE REDUClNG VALVE ION sconce owusuon PUMP DIFFUSION PUMP PATENTED 3.860.848

SHEET IN 2 SAMPLE DEWAR VESSEL PRESSURE ELECTRODE3 x REDUCING VALVES CATHODE|5 SOURCE CHAMBER us COLLECTOR LECTRODES 24 1 l Focusmc; ELECTRODES 2e 8 so? MW T} DIFFUSION /SEPARATING I dmrruslou PUMPS 'TUBE P 60- MAGNET SECONDARY ELECTRON MULTIPHER DIFFUSION PUMP T7 PATENTED 1 1975 SHEET 2 OF 2 SUBSTANCE PRESSURE. REDUCING VALVE coouua FLu|o----- ION SOURCE E D O N A MAGNET 24 A, B,C

E a u T GRID 1 DIFFUSION PUMP DIFFUSION PUMP FIG.2

HIGH PRESSURE ION SOURCE FOR ION OPTICAL ANALYTICAL EQUIPMENT AND FOR PARTICLE ACCELERATORS This invention is concerned with the construction of an ion source, in which a metal gas inlet tube is fitted at one end with the ionization chamber and at the other end with a pressure reducing valve. The cathode of the electron gun is separated from the ion chamber by a beam transmitting system, which has at the cathode a vacuum exit tube. A metal grid for electrostatic screening is fixed around the acceleration path of the ions, which leave the ion chamber. The gas inlet tube, the pressure reducing valve and the screening grid are maintained at about the same potential as the ionization chamber.

This ion source can be operated at gas pressures of about 1 Torr in the gas inlet tube without gas discharges or sparking. Therefore, this high pressure ion source is especially useful as an electron attachment ion source for mass spectrometry.

In general, atoms or molecules of a gas or a solid sample are ionized in an ion source and then accelerated and focused in one direction to produce an ion beam which can be directed into a mass spectrometer or a particle accelerator. Depending on the purpose, it may be necessary to produce atomic ions, as for particle accelerators, or to ionize neutral molecules so mildly that no fragmentation occurs, as for analytical purposes in mass spectrometry.

In mass spectrometry there exist different types of ion sources. A frequently used ion source is the electron impact ion source for production of positive or negative ions which is run at electron energies between 7 and 200 eV. Sample molecules are thereby fragmented making it difficult to analyze mixtures. An ion source which avoids fragmentation is the field ion source or field desorption ion source. This ion source is not stable in service and it can hardly be used for quantitative measurements. In addition, the sensitivity is rather poor compared with an electron impact ion source. Both these ion sources can be operated for gas pressures up to 10 Torr only. For a better detection limit higher gas pressures in the ion source are necessary.

An ion source which can be operated at higher gas pressures up to several Torr is for example the chemical ionization ion chamber. In this ion source a gas, e.g., methane, is ionized by electron impact and the added sample gas becomes ionized by charge transfer from the methane. In addition to pure charge transfer there may occur ion molecule reactions which result in ions differing from molecular ions. Furthermore, it is difficult to determine the partial pressure of the sample in the gas mixture. Thus, quantitative measurements can hardly be performed.

Another ion source which can be operated at gas pressures of several Torr is the electron attachment ion source described by von Ardene (Zeitschrift fur angewandte Physik, Volume 11, 1959, page 121; Tabellen zur angewandten Physik, Volume 1, Berlin 1962). This ion source is either a gas discharge ion source with a permanent gas discharge in a neutral gas, e.g., argon, or a modified electron impact ion source whereby the impact of the neutral gas with electrons at energies slightly above the ionization energy, secondary electrons are produced. In both cases low energy electrons are produced which can attach to molecules and atoms of the sample gas in the gas mixture to give negative ions. Here also, the partial pressure of the sample is difficult to determine and, therefore, this ion source is used for qualitative measurements only.

Thus, there exists a demand for an ion source which is stable in operation at gas pressures above 10 Torr and up to about l Torr. When such an ion source is run as an electron attachment ion source it can be used for quantitative measurements in mass spectrometry, especially for the analysis of multicomponent mixtures. In addition it is possible to investigate the formation of molecular complexes as this occurs during condensation.

A preliminary step towards this goal was published in Zeitschrift fur Naturforschung, Volume 25a, page 849. In this construction the electron beam is especially focused to the far distance towards the entrance hole of the ionization chamber. The beam goes through a tube cooled with liquid nitrogen. This separates the ionization chamber from the hot cathode to prevent back diffusion of pyrolysis products. The electron attachment ion source described by von Ardenne does not employ this focusing for far distances. Furthermore, the preliminary ion source of 1970 accelerates the electrons to such high energies, e.g., 800 eV, that even at higher gas pressures the electrons are not easily deflected from their straight path, so giving a well focused beam. On entering the entrance hole to the ionization chamber, the electrons are slowed down to an energy which still permits multiple ionization, e.g., 200 eV. In contrast to other high pressure ion sources, this one can be run with the sample gas alone and does not demand a neutral carrier gas. However, this ion source was still unstable and only occasionally permitted quantitative measurements at pressures above 10 Torr in the region around the ion source. At these pressures stray currents occurred by gas discharge or sparking; these changed potentials and gave rise to strong fluctuations of the ion current.

Therefore, in order to avoid gas discharge and sparking a reconstruction of the ion source was desirable.

This reconstructed ion source is characterized by the fact that with special potential screening and potential voltages it can be stably operated at high pressures.

The present invention is illustrated in the drawings, in which:

FIG. 1 is a schematic diagram of a mass spectrometer employing the high pressure ion source of the present invention; and

FIG. 2 is an enlarged view of the ion source, including the gas inlet system.

The ion path, which is very sensitive to gas discharge and sparking, is screened against the surroundings at ground potential by a metallic grid 1 connected electrically with the ionization chamber 21. This grid 1 prevents the migration of charge carriers into the vicinity of the ion source 14, where they could produce gas discharge. v

The gas flow goes through a metallic tube 2 which is electrically connected with the ionization chamber 21 includes the metallic pressure reducing valve 9. This metallic tube 2 is electrically isolated from the metallic vacuum wall 12 through which it passes. Instead of a metallic pressure reducing valve 9 a non-metallic one can be used. The other side 8 of the pressure reducing valve 9 is connected with a vessel containing gases, liquids, or solids under their vapor pressures.

The entrance of the electron path into the ionization chamber 21 is supplied with an electrode 3, which extends into the wall of the ionization chamber 21. The evacuation of the liquid nitrogen cooled tube, the anode tube 16, is done by a diffusion pump 30 through a metallic screen 4 close to the cathode 15. The bore of the vacuum line is larger than the diameter of the metallic screen 4 in the anode tube 16. Therefore, the space between the anode tube 16 (which has a potential of, e.g., 2.2 keV) and the outer vacuum wall (which has ground potential) can be kept under high vacuum to prevent gas discharge and sparking.

The stable performance of the invented ion source 14 is obtained by prevention of gas discharge and sparking in those parts of the ion source 14 which must be kept at stable potentials in order to have a constant ion current. For the ion path this is possible by screening the accelerating electrodes 24 and 25 and the focusing electrodes 26 with a metallic grid 1 (which has a potential of, e.g., 2.9 kV positive or negative) from the vacuum walls 18 (which have ground potential). In the gas inlet tube 2 gas discharges at high gas pressures can be prevented by using a metal tube 2 and a metal pressure reducing valve 9, both of which have a potential of, e.g., 2.9 kV positive or negative. Thus the gas is contained in a Faraday cage and no gas discharge is possible. Gas discharges in the electron path are suppressed at one end by an electrode 3 which extends into the wall of the ionization chamber 21 so that the gap is too narrow to allow build-up of a sufficient cathode fall for the ignition of a gas discharge. At the other end of the electron path a large vacuum pump 30 evacuates the gas in the anode tube 16. Therefore, it is possible to' maintain such a good vacuum between the anode tube 16 (with a potential of, e.g., 2.2 kV positive or negative) and the vacuum wall (with ground potential) that the ignition of a gas discharge is prevented.

With the invented construction it is possible to run an electron attachment mass spectrometer. In this case the grid 1 of the ion path is maintained at the negative potential of the ionization chamber 21 and the three other electrodes 24 in the same ion path only a few volts different from this potential. This fact differs from known constructions of electron impact ion sources, which use relative potentials of up to several hundred volts for these electrodes. Low voltage differences are especially important for electron attachment mass spectrometry since the negative ions can only be produced by electron attachment at low electron energies and it is important to prevent even minor stray fields which may come into the ionization chamber 21 from the electrodes 24 and may influence negatively the performance of the ion source 14. This effect is prevented by the above-mentioned electrode potentials.

In running electron attachment mass spectrometry, the equipment shown in FIG. 2 could be stably operated with a sample vapour pressure in the reservoir of l80 Torr acetone, with a gas pressure in the gas inlet tube 8 of 1 Torr, and with a gas pressure in the space between the ion source 14 and the vacuum walls 12 and 18 of 1.5Xl0' Torr. The potential of the gas inlet tube 2 and of the ion path were as follows: 2.9 kV for the ionization chamber 21, the gas inlet tube 2, the pressure reducing valve 9 and the first electrode 24A; 2.895 kV for the second electrode 24B; 2.830 kV for the third electrode 24C and again 2.900 kV for the grid 1. The potentials of the electron path were: 2.1 kV for the electrode 3 extending into the ionization chamber 21, 2.2 kV for the anode tube 16 and --3.l kV for the cathode 15. Under these conditions it was possible to obtain mass spectra of negative ions in which the fragment ions showed much less intensity than the molecular ions. In addition it was possible to get ions of dimer molecules. Furthermore, the pressure dependencies of the ion intensities could be measured in the pressure region between 5X10- and 10' Torr. These pressures were measured in the space between the ion source 14 and the vacuum walls 12 and 18.

Other measurements were done on organic substances such as benzene, cyclohexane, formic acid, acetic acid, methanol, ethanol and others, as well as with water. It could be shown repeatably that the molecular ions as well as the complex ions showed a higher intensity than the fragment ions. One example was'cyclohexane which does not give a negative ion.

What is claimed is:

1. An ion source having an ionization chamber coupled to a gas inlet tube and surrounded by a source chamber having an ion exit port, a plurality of horizontally extending accelerating electrodes positioned adjacent the ionization chamber, between the ionization chamber and the ion exit port in the source chamber, and an electron gun communicating with the ionization chamber, wherein the improvement comprises:

a grid positioned within the source chamber and extending vertically from said accelerating electrodes toward the ion exit port in the source chamber to define an ion exit path for screening the ions from the source chamber, said grid being maintained at about the same potential as the ionization chamher.

2. An ion source as claimed in claim 1 wherein:

said gas inlet tube is metallic and is maintained at about the same potential as the ionization chamher.

3. An ion source in which the electron gun includes an anode tube and a cathode separated from the ionization chamber and arranged at the remote end of the anode tube with a vacuum inlet in the anode tube near the cathode, as calimed in claim 1 including:

a metallic screen across the vacuum inlet to the anode tube.

4. The ion source as claimed in claim I wherein:

said ionization chamber is maintained at a pressure of between about 10 and about 1 Torr.

5. The ion source as claimed in claim 4 wherein:

the space between the ionization chamber and the source chamber is maintained at a pressure of about 5X10 to about 10 Torr. 

1. An ion source having an ionization chamber coupled to a gas inlet tube and surrounded by a source chamber having an ion exit port, a plurality of horizontally extending accelerating electrodes positioned adjacent the ionization chamber, between the ionization chamber and the ion exit port in the source chamber, and an electron gun communicating with the ionization chamber, wherein the improvement comprises: a grid positioned within the source chamber and extending vertically from said accelerating electrodes toward the ion exit port in the source chamber to define an ion exit path for screening the ions from the source chamber, said grid being maintained at about the same potential as the ionization chamber.
 2. An ion source as claimed in claim 1 wherein: said gas inlet tube is metallic and is maintained at about the same potential as the ionization chamber.
 3. An ion source in which the electron gun includes an anode tube and a cathode separated from the ionization chamber and arranged at the remote end of the anode tube with a vacuum inlet in the anode tube near the cathode, as calimed in claim 1 including: a metallic screen across the vacuum inlet to the anode tube.
 4. The ion source as claimed in claim 1 wherein: said ionization chamber is maintained at a pressure of between about 10 4 and about 1 Torr.
 5. The ion source as claimed in claim 4 wherein: the space between the ionization chamber and the source chamber is maintained at a pressure of about 5 X 10 5 to about 10 3 Torr. 